A common component in many semiconductor devices is an analog-to-digital converter (ADC). In general, an ADC operates by receiving an analog input voltage and converting this voltage into a digital representation. Many different types of ADCs are available.
One particular type of ADC is a so-called successive approximation register (SAR) ADC. In general, a SAR ADC includes various components to receive and process the incoming analog voltage and compare it in a successive manner to a reference value to obtain a final value that is the digital output corresponding to the analog input. Typically, a successive approximation ADC includes a comparator having one input to receive the analog input signal and a second input received from a feedback digital-to-analog converter (DAC) that in turn is coupled to receive a digital code from a SAR that in turn receives the comparator output.
This circuit generally initiates conversion by providing an initial code from the SAR to the DAC (e.g., with its most significant bit (MSB) set at one, and all other bits set at zero). The DAC converts this digital code into a corresponding analog voltage, which is then compared to the analog input voltage. Based on the comparison, the MSB of the SAR is either maintained in its set position if the analog input voltage is greater than the initial code, otherwise this MSB is reset. A similar approximation is then performed successively in turn for each less significant bit until all bits have been updated and the corresponding digital code is provided to the DAC and converted for comparison in the comparator.
SAR ADCs are common ADCs for lower speed applications (e.g., less than approximately 5 mega samples per second (MSPS)) at medium to high resolution (approximately 8 bits to 16 bits). In general, they consume less power and silicon area, but still suffer from drawbacks.